The present invention relates to a voltage regulator for a generator, and particularly concerns a voltage regulator suitable for use with an AC generator for charging an automobile battery.
The generator voltage required for sufficiently charging the battery is relatively high at a low temperature, while it is relatively low at a high temperature. Thus, as widely known in the art, it is desirable, for the regulator used with an AC generator charging an automobile battery, to control the retaining voltage level in accordance with the ambient temperature change.
A method introduced in the specification of U.S. Pat. No. 3,718,849 for example, proposes to vary the regulating voltage for a voltage regulator in accordance with the detected temperature at the voltage regulator.
In the above prior art, the ambient temperature at the voltage regulator is detected so as to vary the regulating voltage. However, in this case, only when the detected ambient temperature is relatively high, the regulating voltage is lowered. This is because the above method aims at meeting the requirements of battery charging properties and electrical load (motor, actuator, and so on) properties. The field current of a self-excitation generator at a duty ratio of 100% is however given by EQU I.sub.F =(V.sub.S -V.sub.F)/R.sub.F
wherein I.sub.F represents a field current (A), V.sub.S represents a regulating voltage (V), V.sub.F represents the voltage drop of a power transistor, and R.sub.F represents resistance (.OMEGA.) of a field coil. Therefore, the change of a field current I.sub.F caused by the change of a regulating voltage V.sub.S is relatively small, e.g., in the temperature range of minus 10 to plus 90.degree. C., the change of field current is about 10%.
Apart from the above, a marked tendency to mount an engine in a small space within the automobile is seen lately. Thus, the ambient temperature at the generator is likely to rise high. So there arises a risk that the temperature of semiconductor elements might exceed an allowable junction temperature due to the heat generation by the voltage regulator.